In many areas of the prior art, for example in vehicle or machinery construction and in steel construction, when clamping a connection a defined mounting force has to be applied in order to ensure the operational reliability. In order to avoid damage or accidents, a sufficient pretensioning force has to be maintained during the entire life cycle of the connection. This may change, however, due to operating loads, temperature fluctuations or settling effects. Moreover, monitoring the acting forces, in particular also the transverse forces, may be desirable for checking the connection. Transverse forces may occur due to operating loads transversely to the connection axis, i.e. transversely to the longitudinal axis of the connection element. In order to permit the loading of the connection to be fully monitored, at least axial and transverse forces, but also optionally further variables such as temperature or air humidity have to be recorded by the measuring arrangement. As connection elements are used in many applications, the cost-effective manufacture is an important requirement for this type of product.
In order to ensure the load-bearing capacity of connections, the pretensioning force has to be monitored at regular intervals. This is, however, costly for commercially available screw means after completing the tightening process. Thus in many fields of application it is usual for personnel to monitor the arrangement at regular intervals, without the pretensioning force which actually prevails in the clamping means being known. In this case, either all screw means are retightened with an empirically determined force or preventatively replaced and often fastened with a predetermined tightening torque. In order to avoid unnecessary costs and to increase the reliability of the connection, therefore, it would be advantageous if the maintenance intervals were controlled as required. Maintenance cycles at chronologically fixed intervals also have the drawback that the connections could fail during the cycles. In order to be able to react as required, the axial force of the screw has to be monitored. To this end, various monitoring devices and arrangements are known, to which reference is made below.
Generally, a differential construction which has as a feature the cumulative combination of individual parts with potentially different materials is disadvantageous with regard to the complexity during manufacture and the resulting costs. From a technological perspective, the teaching based on a differential construction often has drawbacks due to the many individual parts in the flux of force and the resulting resilience and spreading due to the production tolerances of the individual components.
In principle, a differentiation is made as to whether the measuring elements or measuring devices are intended to test, monitor and/or record the connection. With different measuring principles, the measurement variables are evaluated during the use and manufacture of the connection. To this end, a transmission of data from the measuring arrangement is required. The suitability for automatic evaluation depends on the output signals of the measuring device.
Several arrangements are able to monitor alterations to the pretensioning force. Thus, in the invention DE 198 54 062 C1 it is proposed to permit the measurement variables to be recorded externally. This invention has the drawback that further elements have to be built into the connection which produce additional resilience, require particular measures and costs during the mounting procedure and the complexity of the construction is high.
Also in WO 2011/020454 A1 which generally refers to a device for determining and/or monitoring a pretensioning force of a screw connection, a sensor produces data dependent on the value of the pretensioning force and/or on an alteration to the pretensioning force and which are able to be electronically exploited. When this teaching is used, the construction has proved to be unsatisfactory as elements have to be positioned relative to one another which is costly.
A device is disclosed in EP 1 764 596 A2 which forwards the measurement results by means of radio technology. By means of the data interface which is already present, such a method is suitable for automatic evaluation. However, the measuring structure is very complex and thus not cost-effective. Additionally, the device is purely limited to the measurement of the pretensioning.
The teaching of U.S. Pat. No. 4,904,132 makes use of an optical output method: when altering the tensioning inside the connection element a display element incorporated in the screw head changes color. In this case, the continuous monitoring proves problematic and impossible for optically implemented systems.
It is proposed in DE 44 21 959 A1 to monitor visually the installation of a screw, whereby the pretensioning may be discretely detected. This proposed solution does not appear to be suitable for the purpose of continuous and automatic evaluation.
The invention disclosed in US 2002/0054809 A1 monitors the pretensioning force based on a mechanical method by means of disk springs and, in the case of failure of the screw, emits an optically readable signal. This teaching has the drawback that a further element has to be built into the connection which causes additional resilience, requires particular measures and costs during the mounting procedure and increases the complexity of the construction.
In principle, three different arrangements may be differentiated for monitoring the clamping force in a connection: the use of external devices, additional elements in the connection and the detection of the clamping force within or directly on the connection element.
Devices are not only known in order to be able to measure pretensioning forces during operation but also those which during the tightening process monitor the axial force prevailing in the connection.
As soon as the desired force is reached during the tightening procedure in the invention disclosed in DE 199 60 040 A1, the disclosed devices emit a signal. After the tightening procedure said devices are removed again and, as a result, are no longer available for measurements on the connection element.
For monitoring the clamping force by devices during operation, an arrangement is disclosed in DE 10 2005 002 592 A1 which is able to monitor the pretensioning force of the screw means. Said arrangement, however, does not remain permanently installed on the screw connection and also requires an operator.
Furthermore, a measuring device is disclosed in EP 1 764 596 A2 which is positioned on the screw head. Due to a hydraulic device and a measuring stick which is located in a bore inside the screw and moves proportionally with the axial expansion, alterations to the tension are detected. Drawbacks here are the complexity of the construction and the size of the device, whereby the potential uses are limited.
When using additional elements in order to measure the alteration to the pretensioning force in connections, the sensor technology is not directly integrated in and/or on the connection element. DE 19854062 C1 discloses a washer which is either inserted between the screw head and the tool or screw nut and tool. Due to the alterations to the thickness of dielectric layers and the resulting changes in capacitance, the prevailing pretensioning force is determined. This invention has the drawback that further elements have to be built into the connection which cause an additional resilience, require particular measures and costs during the mounting procedure and increase the complexity of the construction.
A similar concept is also used in DE 10 2009 043 267 A1. A drawback in these inventions is that the capacitive sensor is not installed in the screw itself which may give a false reading of the measurement.
In DE 39 34 952 A1 a further monitoring arrangement is proposed which contains a force sensor recording the pretensioning force of the connection element in the installed position and mounted adjacent to the connection element. The force sensor is implemented in the form of a spring arrangement and an associated spring path-dependent switching contact. When reaching the desired pretensioning of the connection element, the switching contact is closed. The switching position is monitored for periodically checking the pretensioning force by means of an electrical test circuit. This invention has the drawback that a further element has to be built into the connection which causes additional resilience, requires specific measures and costs during the mounting procedure and increases the complexity of the construction.
Arrangements are regarded as more advantageous, therefore, in which elements for detecting the clamping force are attached inside or directly onto the connection element. Such an approach is disclosed in DE 10 2009 060 441 A1. The sensitive layers on the surface are disadvantageous here. These layers may be easily damaged during the mounting procedure or during operation which leads to a false reading of the measurement values.
Also disclosed in DE 10 2004 038 638 B3 are connection elements with an integrated sensor for determining axial forces. A transducer which has piezoelectric properties integrated in the screw head emits ultrasonic waves which are reflected at the screw end. The same transducer also serves as a sensor in order to record the acoustic signals emitted. With an alteration to the axial tension of the screw, the ultrasonic permeability of the material alters and thus conclusions may be drawn about the pretensioning force still present. A drawback is the susceptibility to interference from the surroundings.
DE 197 10 960 A1 discloses a fastening screw, characterized in that the bearing surface of the screw head is coated with a piezoceramic, electrically insulating and electrically conducting material and the electrically conducting layers may be contacted at a point which is accessible in the screwed state, the signals thereof being electronically forwarded. The sensitive layers on the surface are disadvantageous here. Said layers may be easily damaged during the mounting procedure or during operation which leads to a false reading of the measurement values.
In addition to the disclosed claims for measuring axial forces in tensioning elements, methods and arrangements for measuring transverse forces are also known. Generally force measuring bolts are used here. The difference here is whether these arrangements are also intended to fulfill the purpose of a connection element.
A force measuring bolt with integrated measurement sensor technology is disclosed in EP 1 933 121 A2. Due to the many individual parts and shaped elements required, however, it is a costly construction.
The measurement of mechanical stresses by means of ultrasonic waves which are produced by piezoactuators is carried out in DE 10 2004 038 638 B3. The shear stress inside the bolt alters the ultrasonic permeability of the material which in turn permits conclusions to be drawn about the stress state.
Shear forces occurring in a bolt may also alternatively be determined by means of strain gauges, as shown in the teaching of DE 10 2005 004 060 A1. In the cited arrangement, this is a measuring bolt. The cost when applying this teaching appears to be excessively high for fastening purposes.
Also DE 10 2006 007 385 A1 detects shear forces occurring in a bolt by means of strain gauges and thus does not satisfy any connection purpose.
Suitable within the meaning of the invention are force measuring bolts which also satisfy the object of a connection element and are able to detect loads transversely to the connection axis. DE 101 11 020 A1 discloses such a connection element with a magnetically sensitive element generating a magnetic field and spaced apart therefrom. The proposed connection element is particularly suitable for detecting a force or mechanical stress acting on the seat of a vehicle. The load-bearing capacity of this connection element is, however, reduced by its type of construction as it is slotted over extensive areas.
DE 102 42 256 A1 discloses a force measuring bolt which is provided with a thread and, therefore, may be used as a screw means. A force measuring bushing may be provided for measuring force, said force measuring bushing being integrated in the resilient screw shank. In the case of shear load, a tensile/compressive stress is produced in the shank which is detected by the sensor technology. A drawback is the complex construction which is demanded by the geometric elements required.